1. Field of the Invention
The present invention relates to a magnetic recording medium. In particular, it relates to a magnetic recording medium a magnetic layer of which has good durability, good abrasion resistance, good magnetic characteristics and good surface smoothness, and also a good cleaning effect on a magnetic head.
2. Description of the Related Art
A magnetic recording medium such as a magnetic tape, a magnetic disk and a magnetic sheet is widely used in audios, videos and computers.
The magnetic recording medium may have a magnetic layer which contains, as a reinforcing agent or an abrasive material, non-magnetic powder such as Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, SiC and .alpha.-Fe.sub.2 O.sub.3 so as to improve the durability and the abrasive resistance of the magnetic layer and to clean deposits on a magnetic head.
However, when the non-magnetic powder is added, the magnetic recording medium suffers from decrease of magnetic characteristics due to the decreased packing density of the magnetic powder and deterioration of electromagnetic transformation characteristic due to the poor surface smoothness of the magnetic layer although the durability and the abrasive resistance are improved. In addition, it suffers from clogging of the magnetic head and increase of the noise since the non-magnetic powder partially drops during a tape traveling and the traveling durability is gradually deteriorated.
Recently, increase of a magnetic recording density is intended by using metal magnetic powder or fine iron oxide magnetic powder. It has been desired to improve the surface smoothness and the electromagnetic transformation characteristic of the magnetic recording medium, and the abrasive resistance and the traveling durability of the magnetic layer. However, when the conventional non-magnetic powder is used, the above defects are increased.
It is known to add a dispersing agent such as a phosphate ester, a fatty acid and various surfactants in a step of dispersing the non-magnetic powder in a magnetic paint so as to improve the dispersibility of the non-magnetic powder. However, when such dispersing agent is added, a so-called blooming phenomenon undesirably arises, namely the dispersing agent exudes on a surface of the magnetic layer.
In addition, it is proposed to use the non-magnetic powder which is treated with a coupling agent such as a silane coupling agent and a titanium coupling agent so as to improve the reinforcing effect on the magnetic layer and the dispersibility of the non-magnetic powder. However, the reinforcing effect and dispersibility are not satisfactory since the coupling agent is easily separated from the non-magnetic powder under mixing and dispersing conditions at high shear during the preparation of a magnetic paint.
In addition, it is proposed to improve the magnetic characteristics, the surface roughness, the abrasive resistance and the durability of the magnetic layer by increasing a surface acidity of the non-magnetic powder.
Namely, there is known a magnetic recording medium characterized in that inorganic powder particles are present in a high ratio in the vicinity of the magnetic layer surface since the inorganic powder particles having a Mohs hardness of at least 5 repulses a binder by coating surfaces of the inorganic powder particles with an acidic material or an acidic oxide material so as to increase an acidity (U.S. Pat. No. 4,917,947). There is also known a magnetic recording medium having a magnetic layer which contains magnetic powder and inorganic powder having a Mohs hardness of at least 6, an average particle size of at most 1 .mu.m and pH of less than 7 through coating of an acidic material (Japanese Patent Kokai Publication No. 201822/1989).
However these magnetic recording media provide unsatisfactory effects. Since the surface hardness of the inorganic powder is deteriorated because of the coating material for acidification and the abrasion strength is decreased, the sufficient head cleaning effect cannot be obtained. The coating material for acidification tends to be separated from the inorganic powder during the preparation of the magnetic paint.